something or someone that is regarded as outstanding, extremely good ; the best ; the cream ; the quintessence ; the jewel ; the top ; the bee's knees ; the cat's pyjamas

[UK];[Slang] Refers to the dog's habit of licking its testicles. By extension (and not without humor) the latter probably taste good!
Ex: Among their albums,"master of puppets" is likely the dog's bullocks!